The present invention generally relates to methods and devices for sensing components in a gaseous mixture. Particularly this invention relates to improved methods and devices for sensing hydrogen/hydrocarbons using the electrochemical mixed potential effect.
The automotive industry long has sought to monitor exhaust gases issuing from an internal combustion engine, using sensors to detect and analyze such exhaust gas components as hydrogen (H2), oxygen (O2), hydrocarbons (HC), nitrous oxides (NOx), and the like. Being able to monitor the content of the exhaust gas accommodates numerous beneficial results. For example, such information can be applied to the determination of catalytic converter efficiency, the reduction of emissions to meet state and federal requirements, and the adjustment and optimization of air/fuel ratio for combustion in the engine cylinders.
Using the principles of electrochemistry, one can employ a mixed potential method for sensing hydrogen or hydrocarbons in a gas stream. The principle involved can be expressed by the equation:   E  =                    n        ⁡                  (                                    k              ⁢                              xe2x80x83                            ⁢              T                        q                    )                    ⁢              log        ⁡                  (                      P                          H              ⁢                              xe2x80x83                            ⁢              C                                )                      -                  m        ⁡                  (                                    k              ⁢                              xe2x80x83                            ⁢              T                        q                    )                    ⁢              log        ⁡                  (                      P                          O              2                                )                      +                  (                              k            ⁢                          xe2x80x83                        ⁢            T                    q                )            ⁢              log        ⁡                  (                                    K                              H                ⁢                                  xe2x80x83                                ⁢                C                                                    K                              O                2                                              )                    
where:
E=Emf, electromotive force
n, m, q=constants
k=Boltzman constant
T=absolute temperature of the gas in xc2x0Kelvin
PHC=partial pressure of H2/HC
Po2=partial pressure of O2 
KHC=exchange charge reaction constant of the sensor system to H2/HC
Ko2=exchange charge reaction constant of the sensor system to O2 
As can be observed from this equation, in order to utilize this principle, the electrode/electrolyte system must satisfy the following conditions:
1. none or very little catalytic reaction may occur between hydrogen/hydrocarbon and oxygen at the sensing electrode;
2. the exchange charge reaction rate of oxygen must be less than the exchange charge reaction rate of hydrogen/hydrocarbon; and
3. there must be constant exchange charge rate constants with respect to time and ambient environment.
There have been found numerous materials that exhibit the requisite lack of catalytic reaction to satisfy the first listed condition. Such electrode materials include Pt, Au, Au/Pt alloys, Pd, Pd/Pt alloys, Ag, Ag/Pt alloys, ZnO, CdO, and the like. However, these materials also feature exchange charge rates that limit their utility pursuant to conditions 2 and 3, above. In addition, such materials have serious aging effects and usually exhibit instability towards the ambient atmosphere. A sensing electrode continues to be needed which can satisfy all of the required conditions.
Disclosed herein are sensor elements, methods for fabricating sensor elements, and methods for sensing a gas. One embodiment of a method of fabricating a sensor element for an exhaust gas sensing device, comprises disposing an electrolyte in ionic communication with a sensing electrode and a reference electrode to form the sensor element. The sensing electrode comprises an activator comprising silica and an oxide of an element. The element is selected from the group consisting of alkaline earth elements, rare earth elements, and combinations comprising at least one of the foregoing elements.
One embodiment of the sensor element comprises an electrolyte in ionic communication with a sensing electrode and a reference electrode. The sensing electrode comprises an activator comprises silica and an oxide of an element. The element is selected from the group consisting of alkaline earth elements, rare earth elements, and combinations comprising at least one of the foregoing elements.
One embodiment of the method for sensing a gas comprises: exposing a sensing electrode and a reference electrode to the gas and detecting hydrogen in the gas.
The above described and other features are exemplified by the following figures and detailed description.